Civilians who had been hiding inside the Westgate Mall in Nairobi during the standoff exit the scene Sept. 21. 2013. The extremist group Al-Shabab live-tweeted about this mall siege in Kenya.

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Political extremists, criminals and gang members are advertising their wares, flaunting their exploits and recruiting new members in 140 characters or less, according to law enforcement authorities, criminologists and security experts.

The most shocking example occurred a week ago when the extremist group al-Shabab live-tweeted about the mall siege in Kenya, defending the mass killing, threatening more violence and taunting the military. ...